Ah, now I see your concern.
Project Gutenberg isn't quite your only option any more. Have a look at these:
http://www.ebooks.com/
http://www.fictionwise.com/
You can also get texts directly from
Baen Publishing through their
Webscription service (you choose what you want to get & pay for -- they don't bug you about it), for cheap and in multiple formats. You can even get some of them
before the paper release date.
I know about eReader.com, but they only sell in the format that fits their reading software at the moment. I'm sure there are other sources I don't know about, I'm still pretty new to all this myself.
Perhaps others might suggest more sources for e-texts....